Why Saturn's rings will disappear today

Saturn’s 170,000-mile-wide ring system will disappear today – at least from where we're standing.

According to Nasa, the phenomenon has happened every 15 years since the rings formed as long as 4.5 billion years ago.

Linda Spilker, deputy project scientist for Nasa's Cassini Saturn mission in Pasadena, California, says that it is actually a ring plane crossing illusion or equinox. "The magician's tools required to perform this trick are pure sunlight, a planet that wobbles, and a main ring system that may be almost 200,000 miles wide, but only 30 feet thick."

All of the planets in the solar system "wobble" on their axes. This causes a change of altitude, which will eventually place a planet's equator directly in line with the photons of light streaming in from the sun, according to Nasa. This phenomenon is the equinox.

On Saturn, it occurs twice during each 29-year-long orbit around the sun (so about every 15 years).

"Whenever equinox occurs on Saturn, sunlight will hit Saturn's thin rings, the ring plane, edge-on," says Spilker. "The light reflecting off this extremely narrow band is so small that for all intents and purposes the rings simply vanish."

The Saturn equinox was first recorded 400 years ago by Galileo. In December 1612, he described how he had been studying Saturn and its "two large moons" (through his primitive telescope he mistook the ring system for moons on either side of the planet) for more than two years, and had noticed the "two moons" getting thinner and thinner. After the rings disappeared from his eyepiece entirely, Galileo wrote in a letter: "I do not know what to say in a case so surprising, so unlooked for and so novel."

As Spilker says: "Galileo had every right to be mystified by the rings. While we know how Saturn pulls off its ring-plane-crossing illusion, we are still fascinated and mystified by Saturn's rings, and equinox is a great time for us to learn more."

The equinox, for example, allows the sunlight to hit the rings at 90-degree angles, which can illuminate, or throw shadows, revealing ring structures and oddities previously unseen, says Nasa.

"Around equinox, Cassini's thermal instrument is tasked with measuring the temperature of both sides of the rings as the sun sets to look at how the rings cool as they go through this seasonal change," Nasa says. "The spacecraft's cameras are looking for topographic features in the rings, like tiny moons and possible ring warps, which are only visible at equinox, while the near-infrared and ultraviolet instruments will be on the hunt for signs of seasonal change on the planet."

"The great thing is we are not sure what we will find," said Spilker. "Like any great magician, Saturn never fails to impress."